Feld Hellschreiber

Hellschreiber is a teleprinter system, developed in the 1920s, that transmits text via sound, much like an old facsimile (fax) system. Each pixel of a character is transmitted as a short burst of audio. It works even under very noisy and poor reception conditions, because your brain does the work of analyzing the received image to pick out the characters.
It was used by the German military during World War II to send messages encrypted by ENIGMA machines. Today it is used by amateur (ham) radio operators around the world to send messages to each other via shortwave radio.
Feld Hellschreiber literally means "bright field recorder". Hellschreiber is sometimes referred to as simply Hell.
Feld Hellschreiber lets you decode and transmit Feld Hell messages. Decoded text is continuously scrolled across the top of the display. You can type in text to transmit, or press a button to easily send canned text messages, such as CQ calls. Sound is picked up from your radio's speaker via the internal mic in your device, and transmitted audio comes out the speaker - you can also connect a patch cable to your radio's mic input via the headphone jack on your device to transmit the audio, if you wish.
The slider control adjusts the display contrast, and the indicator to the right shows the volume. Adjust the slider for the best display.
To transmit text, touch on the text entry field, and start typing. Each character will be sent when possible. When sent, it will be taken off the left end of this field, and displayed on the right end of the transmitted text field, to the left of the entry field, in a scrolling fashion.
Canned text buttons are also available, touch one to send the text stored for that button. To edit a button, touch the Setup button, and then select the button you wish to edit. You can edit the name of the button, as well as the text.
To decode transmissions, you just need to run the app, and set it near the radio's speaker or headphones, and watch the decoded text appear. It's that easy!